1. Field of the Invention
The present invention relates to a measuring apparatus to be used in a step of adjusting an optical axis of a headlight of a vehicle such as a motor vehicle, as well as to a method of adjusting the optical axis by using this measuring apparatus.
2. Description of Related Art
As a method of adjusting the optical axis of a headlight, there has hitherto been known the following. Namely, a light from a headlight is projected to, or illuminated on, a screen which is disposed in front of the headlight. That illuminating pattern of the headlight which appears on the screen is pictured by a camera. Then, the optical axis of the headlight is adjusted such that a measuring reference point, which is measured by image processing from the illuminating pattern and which has a certain correlation with the optical axis of the headlight, falls within a predetermined acceptable range (see Japanese Published Examined Patent Application No. 9298/1990 and Japanese Published Unexamined Patent Application No. 103743/1991).
The measuring reference point is set in various manners. For example, a center of gravity of a range of illuminance above a predetermined value within the illuminating pattern is set in some cases as the measuring reference point and, in the case of a cutoff type of headlight, an elbow point which is the crossing point of a horizontal portion and an inclined portion of a cutoff line (i.e., a border line between a light portion and a dark portion) is used as the measuring reference point.
In the above-described prior art, an assumption is made that the headlight is present in a predetermined set position. That acceptable range on the screen within which the measuring reference point is to be positioned is determined by the above-described setting position, a normal direction or orientation of the optical axis and a distance to the screen. However, there are cases where the actual position of the headlight deviates from the set position due to an initial running-in or break-in period of suspension members, variation in air pressure in pneumatic tires, assembly errors, or the like. In such a case, even if the orientation of the optical axis deviates from the normal orientation, the measuring reference point may fall within the acceptable range, with the result that an exact adjustment of the optical axis can no longer be made.
In order to solve this kind of disadvantage, the following method is known in Japanese Published Unexamined Patent Application No. 147030/1992 (corresponding to U.S. patent application Ser. No. 08/031,468) by the inventors inclusive of the inventor of the present invention. Namely, in front of a headlight there is disposed a lattice member which has a plurality of longitudinally elongated lattice holes arranged into matrix. A light beam to be transmitted through the lattice holes is projected to the screen. The illuminating area and the illuminance of the transmitted light through the lattice holes at each of the illuminating regions which are divided into matrix by each of the lattice holes are then measured in order to adjust the optical axis of the headlight. According to this method, there is an advantage in that the position of the light source and the direction or orientation of the optical axis can be accurately measured. On the other hand, there is a disadvantage in that the image processing must be carried out for each of the large number of illuminating regions that are divided into matrix, with the result that the processing is time-consuming and that a cycle time for adjusting the optical axis becomes long.
By the way, the headlight is so arranged that a light beam from a filament, which is the actual light source, is projected forwards through reflection by a reflecting mirror. It can therefore be regarded that the light beam is projected forwards from an imaginary light source on the reflecting mirror. When the headlight in a lighted condition is looked at from the front side with the viewer's eyes half-closed, both the left and the right side portions inside the headlight can be seen bright. These bright portions are considered to represent images of the imaginary light source on the reflecting mirror. When the headlight is looked at from a predetermined position in front of the headlight, the images of the imaginary light source also vary with the change in the position of the headlight.
In view of the above-described finding, the present invention has an object of providing a measuring apparatus in which not only the optical axis of the headlight but also the position of the headlight can be accurately measured in a short time, as well as providing a method of adjusting the optical axis by using this apparatus.